1. Field of the Invention
This invention relates generally to electromagnetic interference (EMI) suppression, and more particularly to an EMI suppressing interconnect to reduce the EMI produced by the commutating action within a direct current (DC) motor.
2. Description of Related Art
Direct current (DC) motors are well-known, and are commonly used in a broad variety of applications. One common application for DC motors is in printers associated with computers and word processors where the printer head is moved transversely along its path across the paper by a DC motor.
The computer industry is known to be very competitive, and is particularly so in the area of computer peripherals such as printers. Accordingly, there is an ongoing effort to reduce the cost of manufacturing printers while maintaining their reliability and compliance with applicable regulations such as those of the FCC and others regarding permissible amounts of electromagnetic interference (EMI) generated by printers. One opportunity to reduce the cost of a printer lies in the use of a less expensive DC motor to drive the printer head. However, lower-cost DC motors generate increased levels of EMI which often exceed those allowed by applicable regulations. The EMI referred to is that generated in the current supply leads by the commutating action of the DC motor as electrical contact between the brushes and successive segments of the rotor is repeatedly made and broken. FIG. 2 demonstrates the problem by showing the maximum, minimum and average EMI levels of a sample of fifty printer head drive motors. The maximum EMI value of the sample exceeds the maximum levels of EMI allowed by regulatory agencies at frequencies between 100 and 200 MHz. Therefore if lower cost DC motors are to be used to drive printer heads, their higher levels of EMI must be reduced.
Various methods are known for reducing EMI associated with DC motors. For example, a resistor ring having a resistance of 200-500 ohms may be fitted to the commutator. In the case of the restrictive packaging requirements of printers, this additional resistance results in the generation of additional heat which must be removed from the printer. Also it is difficult to control the resistance, and results in significant changes in the DC motor performance. Alternatively, a capacitor may be connected across the leads supplying current to the motor, but will not normally result in sufficient attenuation of EMI to meet the desired levels, or a ferrite ring may be fitted the leads to the motor. Both these solutions are also costly.
A varistor may be incorporated into the construction of the motor to reduce EMI, and typically will reduce EMI to acceptable levels. In some cases, however, packaging requirements or additional tooling costs involved in the manufacture of the motor render this solution unacceptable. Varistors are also not retrofittable to a motor.
Accordingly, a need remains for a low-cost DC motor which generates a reduced level of EMI, which is economical to manufacture and which, if need be, can be retrofitted to an existing DC motor.